Introduction

It is well know that human activities have a big influence in our surroundings, from agriculture to urbanization, and the different activities. These changes have different and diverse impacts in the natural environment at all levels.

This relationship has been a subject of research for a long time, some studies were carried out around the 1980's trying to explain these effects (e.g. Sagan et al, 1979; Mintz, 1984), regarding the amount of observation data that supports the idea, quantitative measures were unavailable until the developed of more sophisticated models.

The article "Under what conditions dose land-cover change impact regional climate?" by Y. Xue and M.J. Fennessy, reviews different studies that investigate these impacts in the Sahel, East Asia and central United States.

SAHEL DESERTIFICATION STUDY

During the 1980's serious droughts were recorded in the Sahel and East Asia, these effects were attributed to desertification as one of the reasons. The Sahel area which has predominantly annual grasses with low vegetation and trees has a mean annual rainfall between 150 and 600 mm per year. In this area the water shortage lasted for more than 30 years, although in the 1990's rainfall was somehow higher than in the 80's it was still below the historical average. Even so the maximum rainfall was not reduced very much, this change result in rainfall deficiency in the Sahel. (Xue, 2002) In this decade, different studies with general circulation models (GCM's) were done to analyze the function of land cover changes in regional climate. These studies used simple processes. The limitation and simplicity in the models, and that no big changes were produced in the simulated regional climate, derived to think that further research in this direction was in vain.

Studies during the 1980's done by Chervin and Schneider 1976; Sud and Fennessy 1982 confirmed the discovery of Charney et al (1977) which state that increases in albedo caused reduction in precipitation in semi-arid regions. Studies on the effects of soil moisture and evaporation have also been made, in most of these studies some similarities were found, for example that less initial soil moisture leads to less precipitation, in addition the combination of this two factors were studied by Kioth et al and found that these combined effects is almost the same as the sum of the albedo and soil moisture effects alone. Some other factors were also studies, like atmospheric water content and radiation schemes in land-atmosphere interactions. These studies demonstrate that land surface changes might have significant impact on Sahel climate. (Xue, 2002) These studies have shown that interaction between atmosphere-biosphere is much complex in real world and more parameters and processes have to be taken into account. In this context more sophisticated land-surface models are needed in order to determine the impact of desertification in the Sahel. a better model can only be obtained if all components of energy and water balance are considerate. During several years a number of surface scheme, including realistic representations of vegetation were applied to the Shael. These schemes have been joined with different atmospheric models to conduct Sahel drought studies. With the improvement of model technology, more realistic surface and land surface

models can be done, and this would help in our understanding of the mechanisms of land-atmosphere interactions. (Xue, 2002) In the article presented by Y. Xue, the results of the Center for Ocean-Land-Atmosphere Studies (COLA) GCM (Kinter et al. 1988) coupled with SSIB (XUE et al. 1991). In this model for the numerical simulation, a one degree resolution world vegetation map provided the land-surface conditions (details in Xue et al 1991). Twelve vegetation types were used, all of them recognized by SSIB. For the land-surface degradation simulations, different oil types were used. The degradation areas were chosen based on the world desertification map (UNEP 1992, only source available source at the time). The parameters used for the study are shown in table 1. (Xue, 2002) Table 1. Vegetation parameters for five vegetation types. (Y. Xue and Fennessy, 2002)

* The numbers are for means of June, July and August. ** VSMC is the volumetric soil moisture content.

The previous model was integrated with vegetation maps in order to explore the impact of land degradation on seasonal climate variations and hydrology in the Sahel. It is important to keep the noise level as low as possible, for this effect it is possible to reduce the noise by time averaging over longer time intervals and by increasing the sample size. In this model climatological sea-surface temperatures were used as lower atmospheric boundary conditions over oceans. For each condition a vegetation map was used as well as a degraded vegetation map. (Xue, 2002) In the beginning of the dry season, little rainfall related to land-degradation was recorded or simulated in the Sahel. Rainfall difference was found between the degraded and control simulations. Rainfall is reduced in degraded area, but increased slightly in the south. This difference is consistent with the pattern for dry climate anomalies. Nevertheless the land degradation areas related to rainfall reduction also shifted to the southern area of the Sahel, there was a positive anomaly simulated over eastern Africa, this anomaly was consistent

with the rainfall anomaly observed. This effect of the land degradation is not limited to summer rainy season and also not limited to Sahel but it extends to East Arica region during autumn (Y. Xue, 2002). During the July-August-September simulation, temperature in the degraded area was higher than the control, the surface air temperature was increased by 0.8 K, this value is close to the observed 1.1 K increased in the same area. Because of the lower surface roughness, wind became stronger, and this would increase wind erosion.

With the reduction of rainfall, soil moisture, surface runoff and subsurface drainage were consistently reduced in the degraded areas. The simulated river discharges from control and degradations experiments at different stations were compared with observations, the difference in river discharge between control and degraded simulation correspond to the observed difference in the data series. Due to limitation in the grid size, it is possible to only examine a macro-hydrological process. (Xue, 2002) Because of the lack of data to identify the extend of degree of Sahel desertification, other sets of experiments were designed to investigate how the land-surface changes might affect the results. Five different regions were degraded; in general, degradation resulted in reduction of rain fall, on the degraded areas. However, large differences in the impact in the different areas showed that the location and extend of the degradation area was important. (Y. Xue, 2002) Some examples of this change are that the degradation in the northern Sahel and the West African Sahel resulted in the largest and most significant reductions of rainfall. On the other hand, degradation of the northern Sahel caused a widespread reduction of rainfall in the northern tropical Africa, both inside and outside the degraded area. In the coastal zone, deforestation caused less impact on the rainfall.

In the experiments, vegetation types along with the vegetation and soil properties were changed. In the SSIB there are around twenty different parameters, the experiments were done by changing individual parameters of a set of vegetation in the degraded areas. This experiments showed which parameters have the largest impact in simulation, the parameters are: surface albedo, surface aerodynamics resistance, stomatal resistance, LAI and hydraulic conductivity. (Xue, 2002) Land degradation also impacted the simulated large-scale circulation and wave propagation. One of the most relevant features of the sub-Sahel drought is the displacement of the intertropical convergence zone (ITCZ) in the dry years. Observation showed that rainfall in dry years was not much reduced.

In summary the impact of the land surface on the Sahel regional climate, the changes in albedo and initial soil moisture from different experiments are shown in table 2. The corresponding change in precipitation is also given. The different GCMs predicted very different mean precipitation Sahel. It is shown in the table that even with the wide range of different surface models; from very simple to complex models, the results for the northern part of African studies are consisted: albedo, soil moisture changes produce negative (positive) results. Even the ranges of results are within a close range. (Xue, 2002) Table 2. Comparison of different Sahel experiments.(Y. Xue and Fennessy, 2002)

EAST ASIAN DESERTIFICATION STUDY

Numerical simulation to determine the relationship between land-surface degradation of Mongolian and Inner Mongolian grassland and the drought in East Asia has also been studied. Few studies were made in this region to investigate the relation between land surface and atmosphere, this due to high internal variability in this region in real world and in model simulations. The results from a ensemble of six 3-month integrations with the couplet COLA GCM/SSIB model is shown. (Xue, 2002) In the simulation, the vegetation of the degraded area was changed from grassland to bare soil (Type 7 to Type 11). The degraded area in the experiment was based on a desertification map compiled by Zhu et al (1988). The period used was the summer monsoon (June to August) using different initial atmospheric conditions. The results showed that degradation of grassland could have a substantial impact on regional climate and over some factors in East Asian summer monsoon.

In southern China and northern China the precipitation has generally decreased over the last forty years. A pattern known as the "sandwich pattern" was observed: negative changes to the south and north of the Yangtze River and positive change in between (N-P-P). (Xue, 2002) In the June-July-August period, the rainfall observed in the 1980s and 1950s and the simulated rainfall have differences between the desertification runs and the control runs. The N-P-N pattern is simulated, but the result is titled to the southwest of that observed. The rainfall reductions occurred within the desertification area, but also to the south of it.

The surface air temperature over the desertification area was higher in the experiment. The increase in temperature in the atmosphere occurred only at low atmospheric levels. Based on observations in China, it was found that the surface air temperature increased yearly during the past forty years, mainly in winter. In summer the surface f most China became cooler. Nevertheless in the Inner Mongolian grassland, the summer air temperature was warmer. (Xue, 2002)

In this desertification areas, during the desertification simulations, soil moisture and surface evaporation was substantially reduced, this has similar results as in Sahel desertification experiments. The change in surface albedo, reduce vegetation coverage, reduce soil water content and reduce in surface roughness resulted in reduction of evaporation. Other observations are that with the reduce of the precipitation, soil moisture decreased, surface runoff increased, however, this increase was due to land degradation. (Xue, 2002) US LAND-COVER CHANGE STUDY

In U.S. the Great Plains area is frequently with drought, but the most notable events were in the 1930s, 1950s and 1980s. The Great Plains region land-cover changed due to human activity. Agricultural settlement of the semi-arid plains over the past century changed the flora, fauna, hydrological and climatological pattern and conditions. In the 1930s the Dust Bowl was one of the best known drought environmental disasters in the U.S. history. Even though this event inspired major advances in soil conservation techniques, in the Great Plains, continued degradation is a mayor issue. A sensitivity study to determine the impact of land-cover changes over the agricultural region in central U.S. on North America regional climate was done. (Xue, 2002) Two land-cover conditions were assigned to the agricultural area over the Great Plains. For one of the simulations, the crop cover, LAI, and vegetation greenness were specified close to satellite-observed values. On another simulation, field measurements at a single crop site after harvest were used to specify land conditions. Since the vegetation cover and LAI were very low after harvest, there was an artificially created degradation in the simulation.

In a three-month simulation from June to August, the simulated seasonal mean temperature was 1-4 K higher and the rainfall was 15-30 mm lower in the degraded areas. Surface evaporation and soil moisture were also reduced. These changes were persistent during the entire simulation. The degradation also influenced the humidity and temperature in the lower troposphere. (Xue, 2002) UNDERSTANDING THE MECHANISMS

Specific land degradation caused anomalies in the simulated climate, which affected the atmosphere, mainly changing the hydrological processes and energy balance at the surface. This processes change because the surface albedo change caused by the land degradation, this has a strong effect on the surface energy balance, and has to be specified realistically. Even though the importance of the albedo, changes in vegetation and soil properties also contribute to rainfall and surfaces changes. (Xue, 2002) Evaporation in Sahel, East Asia and central U.S. was substantially lowered because of a higher surface albedo, lower LAI and higher stomatal resistance; these factors in degradation simulations were consistent with the simulated rainfall reductions. However, other components of the surface energy and water balance changed in a different way across the three regions. Other consequences were that more shortwave radiation was reflected from the surface as a result of higher albedo, but this loss was partially compensated for increased incoming shortwave radiation, this caused by less clouds in the drier atmosphere. (Xue, 2002)

Changes in surface sensible heat flux were associated with the changes in shortwave radiation. The surface sensible heat flux was reduced when shortwave radiation decreases, otherwise it increased in the surface energy budget.

The net longwave radiation at the surface was reduced because the higher surface temperature increased the outgoing longwave radiation, and the reduced cloud and water vapor in the degradation simulations lowered the incoming longwave radiation at the surface. (Xue, 2002) The southwest monsoon flows from Bay of Bengal and The Atlantic Ocean are the main summer moisture sources for East Asia and the Sahel. Studies in both regions showed that lad degradation result in weaker monsoon flow and affected the atmospheric circulation. The affected areas are considerably beyond the degraded specified areas.

CONCLUSION

It is shown that land-cover changes are important aspects of the observed climate anomalies in East Asia and the Sahel. Scientific investigation in these regions was made because of a possible devastating social and economical consequence induced by the future droughts. Different views about the droughts were develop. One states that the main cause is land-surface degradation, this as a result of population pressure (expansion) in the region. Other view indicates that the droughts are due to anomalous patterns in sea-surface temperature. In views, mitigation development and adaption strategies are needed. (Xue, 2002) Sahel area has been studied for more than two decades; results have shown that land-cover degradation has significant impact on the regional climate. First's models of the area were mainly sensitivity studies. When coupled atmosphere-biosphere models were introduced for land degradation-atmosphere interaction, results revealed that experiments not only tested sensitivity, but were also able to link land-surface processes to observed decadal climate changes.

When a coupled model was used, both summer and fall anomalies observed over the African continent and summer patterns in East Asia were simulated in land degradation experiments. This shows that coupled climate-biosphere model may be able to provide regional climate change resulting from land degradation. But before these models can be used for these applications, more validation models and comparison are needed. (Xue, 2002) In both East Asia and Great Plains simulation studies, vegetation conditions did not vary with the climate conditions. Lately additional level of complexity has been explored by coupling of dynamic vegetation models to atmospheric models. With the development of these models, we will be able to asses more realistically the role of sea surface temperatures, land-surface processes and other long term Sahel drought.

Desertification usually occurs in patches; uniform desertification in previous experiments did not represent the real situation and might exaggerate the impact of the land degradation. Regional or mesoscale models are necessary to assess more realistic consequences of desertification in local scales; they will help not only in specifying land degradation conditions but also in the transfer of the information for large scale global models to regional or local scales. (Xue, 2002)

Specification of land degradation are crucial, it is not completely clear how well normal and degraded vegetation scenarios are specified in Sahel and East Asia studies in comparison with the real land-surface conditions in the 1950s and 1980s. With the use of satellite images it will be possible to provide additional information regarding land degradation.

In summary if changes in vegetation properties that occurred between the 1950s and 1980s are comparable to the specified control/desertification differences, the results indicate that anthropogenic land degradation could lead to regional climate changes. To avoid these consequences the implementation of sustainable resource management policies must be a priority. (Xue, 2002) Additional Information

In September 2004 Pitman and Pielke, made a study about the Impact of land cover change on the climate of southwest Western Australia. The issue is that in the last decades a significant reduction in rainfall had occurred in Western Australia. The rainfall reduction leads to a reduced stream flow, warmer daytime and nighttime temperatures. One of the main reasons attributed to this rainfall change is due to greenhouse effect, and a secondary factor might be the extensive land-use changes that have taken place in Australia since the 18th century. (Pitman, 2004) In order to try to explain these changes, modeling is used to asses and determine the impacts of local land-use change on the regional climate. In 2003 Narisma and Pitman explored the impact of continental-scale land cover change on the Australian climate. A high resolution regional climate model was used (MM5). (Pitman, 2004) Since results from a single regional climate model might be not totally truth, three additional models were used. All model simulations are initialized with the same boundary conditions taken from the NCEP Global Data Assimilation System. The models used are MM5, LRAMS and GEMRAMS, a total of 10 simulations with each model was run. The data used in the simulations is data in which El nio and La nia conditions are present, to explore if they have an impact in the simulations. (Pitman, 2004) The simulations gave the following results; the impact caused by the land cover change is due to modification in the vegetation characteristics, such as converting trees to grass, this effect reduces LAI, increases albedo, decreases roughness and alters root distribution and depth. This changes affects the partitioning of available water, therefore it affects the soil moisture and possible rainfall. These changes also affect in the energy balance between sensible and latent heat, his affects the local air temperature and boundary layer structure. Changes in roughness will affect in the wind patterns and turbulence. (Pitman, 2004) In the overall discussion it is concluded that the decline in rainfall in the region is associated with a change in the large-scale atmospheric circulation, and that the warming in the region is associated with the enhanced greenhouse effect, but tin order to demonstrate that the land cover change has a direct influence in the rainfall and regional climate change, a larger model should be developed using longer data series and accurate vegetation change maps and include this change in the models, to determine its influence. (Pitman, 2004)

REFERENCES

Y. Xue and M.J. Fennessy. 2002. Under what conditions does land-cover change impact regional climate?

A.J. Pitman et al. 2004. Impact of land cover change on the climate of southwest Western Australia